The invention is concerned with the measurement of proximity effects which occur in the fabrication of solid state microelectronic circuits by means of a technique known as electron beam lithography. In this technique a pattern of electronic circuits is formed on a mask or directly onto an oxide coated silicon wafer by means of an electron beam which is scanned over the surface of the mask or wafer. The pattern to be lithographed is stored in a computer memory and the computer controls the beam intensity or energy deposit in accordance with the stored pattern program. The wafer or mask is coated with an electron resist material which is exposed when it is impinged by the electron beam. Subsequent development of the mask or wafer will either remove the exposed or the unexposed portion of the resist layer depending on whether the resist is a positive or negative one.
Very large scale integrated circuits of this type necessarily involve closely spaced patterns which require accurate location of electron resist pattern edges. The location of these pattern edges is a function not only of the beam location but also of the proximity effect of the particular wafer or mask being lithographed. The electron beam as it strikes and penetrates the electron resist material will be forward scattered by the molecules of the resist material and as it strikes the underlying silicon dioxide layer it will be backscattered through the resist layer again. This scattering of the electron beam causes the exposed area to overlap the beam edge. The amount of this overlap is a function of numerous factors such as the beam current density, the width of the line or area being lithographed, the resist material and the thickness thereof and the substrate material. Thus the magnitude of the proximity effect varies with the wafer or mask being lithographed and also with the characteristics of the lithography machine. If the magnitude of the proximity effect can be measured, it can be compensated for by modifying the computer software which controls the lithography process. The present invention provides a means and a method of rapidly and accurately measuring this proximity effect.